Physical Sciences Research Highlights

Following Nature's Lead: Mimicking Enzymes to Release Energy

A new catalyst developed by PNNL researchers actually performs best in water and at temperatures and acidities remarkably similar to conditions found in fuel cells. Their paper was first made available online and has since inspired cover art for a future 2016 issue of Dalton Transactions.Enlarge Image.

Efficiently
releasing stored chemical energy harnessed from renewable sources remains one
of the great scientific challenges facing the catalysis research community. Meanwhile,
Mother Nature performs this transformation with ease using abundant metal-containing
enzymes as catalysts. So far, lack of an inexpensive and stable catalyst has
limited widespread, economical use of hydrogen fuel cells (HFCs). But thanks to
a recent breakthrough at Pacific Northwest National Laboratory (PNNL), that may
change.

Researchers at PNNL have
demonstrated that stored renewable energy can be interconverted efficiently and
inexpensively by mimicking enzymatic catalysts used in biological processes.
Enzymes consist of an active site-a metal where the reaction happens with connections
to the rest of the protein-and a protein scaffold surrounding the active site.
That PNNL research team, led by Dr. Wendy Shaw, predicted that many parts of
the protein scaffold play critical roles in catalytic activity and efficiency
instead of only the active site. This protein scaffold is known as the outer
coordination sphere (OCS). It controls the reactivity of the active site by
controlling the movement of substrates during catalysis.

Shaw and her research grouphave shown that adding a simple, amino
acid OCS around an artificial nickel-based catalyst has unparalleled
improvement in performance. The researchers' new catalyst actually performs
best in water and at temperatures and acidities remarkably similar to conditions
found in fuel cells.

Why It Matters: "Overall,
our research shows that proper bridging of synthetic catalysts and features
from natural enzymes can help us develop novel sets of materials that can have
activity far beyond any natural enzymes," said Shaw. "They also perform better
under demanding conditions."

Enzymes are large protein molecules
found in nature that catalyze reactions quickly and efficiently. They are
ubiquitous in all niches of the biosphere, and their roles are clearly evident
in reactions that fuel the natural world, such as photosynthesis and respiration.
In nature, hydrogen (H2)
molecules store energy and release it as needed with the aid of hydrogenase
enzymes. The basic reaction catalyzed by the hydrogenases is the interconversion
of H2 molecules and protons and electrons (H2 ⇔ 2H++2e-).

Although Shaw and her team drew
inspiration from hydrogenases, these enzymes are difficult to produce in large
quantities. They also perform only under a narrow set of conditions, making
them challenging to use in energy applications. But molecular electrocatalysts-inspired
by the same natural enzymes-can overcome deficiencies and provide alternatives.

Some of
the best and most-studied molecular catalysts for H2
activation contain an enzyme-inspired active site. They are a series of
nickel-based catalysts developed at the Center for Molecular Electrocatalysis, an Energy Frontier Research Center at
PNNL. To understand the role of the protein scaffold in enzymes, Shaw's team incorporated
an enzyme-like OCS to these well-studied catalysts.

Interestingly, including just a
single amino acid in the OCS induces water solubility for the catalyst.
Solubility makes a catalyst more versatile and active under a range of
conditions. It also allows researchers to explore speed and efficiency,
including changes in solvent, pressure, and temperature. The researchers found
that the best conditions for operating the catalyst were strongly acidic (pH =
0) and hot (72° C)—remarkably similar to the operating environment within HFCs.

Methods: Shaw and her
PNNL research team assumed they could improve catalytic reactivity. But they
were surprised and excited to learn to what degree.

"Our hypothesis was that we
could include enzyme-inspired features, such as amino acids, tactically around
the synthetic complex and improve its catalytic reactivity," said Shaw. "What
really surprised us was how just changing the solvent from water to methanol
while using the same temperature and pressure resulted in reactivity almost 4
orders of magnitude slower and with significantly less efficiency," added Shaw.

Their results imply that
interactions with the solvent, even similar solvents such as methanol and
water, have a very large influence in controlling reactivity. Differences in
reactivity as a function of solvent will help to unravel how these complexes
operate so efficiently under some conditions.

Electrochemistry and nuclear
magnetic resonance (NMR) spectroscopy were the two primary techniques used
during this study. Cyclic voltammetry allowed researchers to measure catalytic
rates and energy efficiency (overpotential). The team used NMR to probe the
structure of the molecule. Computational studies were used to quantify the
solvent's role in catalytic reactivity.

What's Next? Shaw's team
and international collaborators will seek similar activity at lower H2
pressure and evaluate long-term stability of their catalyst. They hope to test
it in a real fuel-cell setup. Such testing could pave the way for the
development of fuel cells based on inexpensive metals that could replace
platinum-based fuel cells currently in use. This advancement holds tremendous potential
for inexpensively interconverting energy. It could lead to an inexpensive,
environmentally friendly, energy-harvesting procedure for use across the globe.

Acknowledgments

Sponsors: This work was funded by the
Office of Science Early Career Research Program through the U.S. Department of
Energy (DOE), Office of Science, Office of Basic Energy Sciences, and the
Center for Molecular Electrocatalysis, an Energy Frontier Research Center
funded by DOE, Office of Science, Office of Basic Energy Sciences. PNNL is
operated by Battelle for DOE.

Research Team: Arnab Dutta, former PNNL scientist now at Indian
Institute of Technology Gandhinagar, India; Bojana Ginovska, Simone
Raugei, and Wendy Shaw, PNNL; and John A. S. Roberts, former PNNL scientist now at REC Silicon,
Moses Lake, Wash.